1. Field of the Invention
The present invention generally relates to a fluorescent X-ray analyzer and, more particularly, to the fluorescent X-ray analyzer of a type provided with a collimator for restricting the field of view so that fluorescent X-rays and/or scattering X-rays generated from sources other than a sample piece being analyzed will not reach a detecting means.
2. Description of the Prior Art
Generally in a fluorescent X-ray analyzer utilizing parallel beams, as shown in FIG. 19, a sample piece 1 fixedly supported on a sample table is generally radiated with primary X-rays 3 generated from an X-ray tube 4 to excite the sample piece 1. The sample piece 1 so excited generates fluorescent X-rays 5 which are subsequently guided through a collimator 10A to a solar slit 7 to extract the fluorescent X-rays 5 in the form of parallel beams. The fluorescent X-rays 5 emerging from the solar slit 7 are subsequently monochromatized by a monochromator 8 according to spectrum of different wavelengths corresponding to elements contained in the sample piece 1 with a detector 9 consequently detecting the monochromatized X-rays.
The collimator 10A disposed between the sample piece 1 and the solar slit 7 is employed for the following reason.
Since the primary X-rays 3 emanating from the X-ray tube 4 irradiate not only the sample piece 1 to be analyzed, but also the sample table 2, the absence of the collimator results in that as shown in FIG. 20A, not only the fluorescent X-rays generated from the sample piece 1, but also a large amount of disturbing rays such as fluorescent X-rays and/or scattering X-rays generated from a portion 2a of the sample table 2 adjacent and surrounding the sample piece 1 impinges upon the detector 9. For this reason, the disturbing rays eventually constitute a background with respect to the spectrum of the fluorescent X-rays emanating from the sample piece 1, accompanied by reduction in the S/N ratio.
In contrast thereto, the use of the collimator 10A results in that as shown in FIG. 20B, the field of view, or coverage, of the detector 9 is restricted to a portion 2a of the sample table 2 surrounding the sample piece 1. Specifically, this portion 2a of the sample table 2 encompassed by the collimator 10A shown in FIG. 20B, although constituting a source of the disturbing rays, is limited to an area smaller than the portion 2a covered by the detector 9 when no collimator 10A is used, and therefore, the most of the disturbing rays are intercepted by the collimator 10A and does not reach the detector 9, resulting in improvement of the S/N ratio.
The collimator 10A discussed above is of a structure including, as shown in FIG. 19, an oblong plate member having a plurality of, for example, three field-limiting apertures 12a, 12b and 12c of varying diameters defined therein in a row conforming to a direction of sliding motion of the collimator 10A shown by the arrow Y The apertures 12a to 12c are selectively brought into alignment with the path of travel of the fluorescent X-rays 5 towards the solar slit 7 one at a time depending on the size of a target area of the sample piece 1 to be measured.
However, as shown in FIG. 20B, there is a certain distance L between the collimator 10A and the sample piece 1. Accordingly, even though one of the apertures, for example, the aperture 12a of a diameter substantially equal to the size of the sample piece 1 is selected, the xe2x80x9ceyexe2x80x9d of the detector 9 looking at the sample piece 1 through the selected aperture 12a is such that even the disturbing rays emanating from that portion 2a closely exteriorly surrounding the sample piece 1 are apt to pass through the selected aperture 12a and then to be incident upon the detector 9, eventually resulting in incapability of improving the S/N ratio.
If in an attempt to prevent the disturbing rays from entering the detector, one of the apertures which is of a diameter smaller than the size of the sample piece 1 is selected, the intensity of the fluorescent X-rays passing through the selected aperture will decrease, resulting in reduction in sensitivity of detection.
In order to alleviate the above discussed problems, the Japanese Patent No. 2,674,675 discloses the use of such a collimator 10B as shown in FIG. 21. The suggested collimator 10B comprises a plurality of, for example, three tubes 15a to 15c having varying inner diameters each corresponding to the size of a target area of the sample piece 1 to be measured and, also, varying lengths with the largest length chosen for the smallest inner diameter of the tube and the smallest length chosen for the largest inner diameter of the tube such that when one of the tubes 15a to 15c is selected, the smaller the inner diameter of the tube, the closer the tube is to the sample piece 1. In such case, if one of the tubes 15a to 15c in the collimator 10B is properly selected according to the particular size of the sample piece 1 to be analyzed, the xe2x80x9ceyexe2x80x9d of the detector 9 looking at the sample piece 1 through the selected tube is such as to encompass only the sample piece 1 and, therefore, the disturbing rays emanating from an area other than the sample piece 1 can be effectively intercepted. Therefore, with no need to reduce the diameter of the corresponding aperture down to a value smaller than the size of the sample piece 1, any possible entry of the disturbing rays emanating from the area other than the sample piece 1 to the detector through the solar slit 7 can be effectively minimized, accompanied by improvement in S/N ratio without the sensitivity of detection being decreased. However, the use of the collimator 10B has been found having such a problem that as shown in FIG. 22, when the fluorescent X-rays 5 emanating from the sample piece 1 are partially cut off by, for example, the tube 15a, a portion 5a of the fluorescent X-rays 5 emanating from the sample piece 1 tend to impinge upon an inner wall surface 16a of the tube 15a, resulting in generation of disturbing rays 30, such as fluorescent X-rays and/or scattering X-rays, from the inner wall surface 16a which will eventually enter the detector through the solar slit 7. Accordingly, the S/N ratio cannot yet be improved sufficiently. Also, respective longitudinal axes of those tubes 15a to 15c must extend parallel to the solar slit foil and, thus, a high processing precision is required to manufacture the collimator 10B, accompanied by reduction in workability.
On the other hand, in the prior art fluorescent X-ray analyzer, as shown in FIG. 21, the sample piece 1 is irradiated by the primary X-rays 3 with the direction of an axis of the X-ray source 4 inclined relative to the sample piece 1, so that the fluorescent X-rays 5 generated from the sample piece 1 while the X-ray source 4 is positioned as close as possible to the sample piece 1 to cause the latter to receive an increased radiation intensity can enter the detector. The radiation intensity of the primary X-rays 3 towards the target area of the sample piece 1 to be measured means the total radiation intensity of the primary X-rays 3 towards the entire target area of a sample surface 1a to be measured. As a result of the simulated test conducted to determine a pattern of distribution of radiation intensity of the primary X-rays 3 over an imaginary irradiation plane including the sample surface 1a and its plane extension, inclination of the direction of the axis of the X-ray source 4 has resulted in that as shown by a curve B in FIG. 23, distribution of the radiation intensity was not maximized at a location C where the radiation center axis of the X-ray source 4 extends to the imaginary irradiation plane, but was maximized at a location M displaced in a direction conforming to the direction of inclination of the X-ray source 4, thus representing an asymmetric pattern of distribution of the radiation intensity.
Accordingly, assuming, for example, that the target area of the sample piece 1 to be measured has a relatively large diameter D3, the radiation intensity (corresponding to the hatched surface area) of the primary X-rays 3 projected onto the target area of the sample piece 1 to be measured will be maximized at a location shown by D3 whereas if the target area of the sample piece 1 to be measured has a relatively small diameter D1, the radiation intensity thereof will be maximized at a location shown by D1. Thus, depending on the size of the target area of the sample piece 1, the location at which the radiation intensity is maximized varies. Accordingly, the inventor of the present invention have suggested in the Japanese Patent Application No. 8-312673 (U.S. patent application Ser. No. 09/127,724), the use of a movable sample table 2 that can be moved to an optimum position at which the radiation intensity of the primary X-rays 3 towards the target area of the sample piece 1 can be maximized to thereby maximize the efficiency of utilization of the primary X-rays 3 from the X-ray source 4.
However, any of the prior art collimators 10A and 10B discussed hereinabove is so designed that the location C where the radiation center axis of the X-ray source 4 extends to the imaginary irradiation plane may be chosen as a location where the collimators 10A and 10B work to partially cut off the fluorescent X-rays 5. In other words, even though any of the apertures in the respective collimators 10A and 10B is chosen, the center at which the aperture encompasses necessarily lies at a certain location C. On the other hand, as discussed above, if the target area of the sample piece 1 to be measured is moved to the optimum position, the position M at which the intensity of X-ray radiation is maximized is displaced from the location C and, therefore, if the center at which the aperture encompasses lies at the location C, the fluorescent X-rays 5 emanating from the target area to be measured will not be sufficiently impinge upon the detector, consequently, the S/N ratio will not be sufficiently improved.
Accordingly, the present invention is intended to provide an improved fluorescent X-ray analyzer equipped with a collimator which is effective to improve the S/N ratio as compared with that exhibited by the prior art collimator, without accompanying any possible reduction in sensitivity of detection and which is excellent in workability.
In order to accomplish the above described object of the present invention, one aspect thereof provides a fluorescent X-ray analyzer which comprises a detecting means for detecting fluorescent X-rays emitted from a sample piece to be analyzed, and a first collimator disposed between the sample piece and the detecting means and supported for movement between inserted and retracted positions with respect to a path of travel of the fluorescent X-rays towards the detecting means. The first collimator comprises a wall adjacent the sample piece that is stepped to provide stepped wall segments having respective apertures of varying diameters defined therein. The smaller the aperture, the closer it is to the sample piece when one of the apertures is selected according to a size of a target area of the sample piece to be measured and is then brought in register with the path of travel of the fluorescent X-rays towards the detecting means.
According to the construction described above, by suitably selecting one of the aperture according to the size of the target area of the sample piece to be measured, since only the sample piece can be encompassed when the sample piece is viewed from the side of the detecting means, as is the case with the collimator described in connection with the prior art, any possible entry of the disturbing rays, generated from other than the sample piece, into the detecting means can be prevented and, therefore, the S/N ratio can be improved without the detection sensitivity being lowered. Also, since the path of travel of the fluorescent X-rays passing through any of the apertures is not of a type having a narrow width surrounded by an inner wall surface, the disturbing rays will hardly occur from the inner wall surface and, hence, the S/N ratio can further be improved as compared with the prior art.
Preferably, the first collimator comprises a substrate and a projection formed on the substrate, said projection having a tip where the stepped wall segments are formed.
Also preferably, a second collimator is disposed between the first collimator and the detecting means and supported for movement between inserted and retracted positions with respect to the path of travel of the fluorescent X-rays towards the detecting means. This second collimator has one or more apertures of a diameter larger than that of any one of the apertures in the first collimator. According to this structure, since one or more apertures of the diameter larger than that of any one of the apertures in the first collimator, which are not required to be approached towards the target area of the sample piece to be measured are defined in the second collimator which is a member separate from the first collimator and since the second collimator is positioned rearwards of the first collimator, as compared with all of the apertures are arranged in the first collimator, the length in a lateral direction can be reduced, making it possible to render the collimator in a compact size.
Again preferably, a drive mechanism is provided for moving the sample piece to an optimum position at which a radiation intensity of a primary X-rays towards a target area of the sample piece to be measured can be maximized depending on the size of the target area of the sample piece. In such case, the apertures in the first and second collimators are arranged so as to encompass the target area of the sample piece as viewed from the detecting means. According to this structure, since the apertures in the first and second collimators are so arranged as to encompass the target area to be measured then held at the optimum position at which the radiation intensity of the primary X-rays is maximum, the fluorescent X-rays generated from the sample piece can be sufficiently utilized to further improve the S/N ratio.
An another aspect of the present invention provides a fluorescent X-ray analyzer which comprises a detecting means for detecting fluorescent X-rays emitted from a sample piece to be analyzed, and a first collimator disposed between the sample piece and the detecting means. The first collimator has a plurality of apertures defined therein, and also has a flat wall adjacent the sample piece, at least a portion of said flat wall being positioned within an area of irradiation of primary X-rays generated from a X-ray source, and a shielding wall for preventing the primary X-rays, generated by the X-ray source, from entering a path of travel of the fluorescent X-rays from the side of the detecting means of the wall adjacent the sample piece.
According to this construction, since the wall adjacent the sample piece is flat, it can easily be processed. Also, since the provision has been made of the shielding wall for preventing the primary X-rays, which would serve as disturbing rays, from entering the path of travel of the fluorescent X-rays, the disturbing rays can be prevented with the S/N ratio improved consequently.
A further another aspect of the present invention provides a fluorescent X-ray analyzer which comprises a detecting means for detecting fluorescent X-rays emitted from a sample piece to be analyzed, and a first collimator disposed between the sample piece and the detecting means. The first collimator comprises a flat wall adjacent the sample piece and having a plurality of apertures defined therein, and a shielding portion provided at a front side of said flat wall for preventing primary X-rays, generated by a X-ray source, from entering any one of the apertures.
According to this construction, since the shielding portion serves to prevent the primary X-rays, which would constitute disturbing rays, from entering any one of the apertures, the S/N ratio can be improved.
Preferably, the sample piece is supported by a sample table and the first collimator is arranged so as to receive fluorescent X-rays in a direction inclined relative to the sample table. The first collimator has a corner area confronting the sample table, which is cutout to define a cutout surface parallel to a surface of the sample table. According to this structure, any of the apertures in the first collimator can be brought close to the sample table without being interfered with the sample table accompanied by improvement of the S/N ratio.
Also preferably, the shielding portion may be either a visor formed integrally with the first collimator or a visor formed by fitting a separate plate to the first collimator. Further preferably, the visor of the first collimator has an extension which is formed with a primary X-ray aperture. According to this structure, since the primary X-rays partially cut off by the primary X-ray aperture impinges upon the sample piece, any possible generation of the fluorescent X-rays from around the sample piece can advantageously prevented, resulting in increase of the measurement accuracy.
Furthermore, the shielding portion is preferably arranged to incline in a direction in which a portion of the wall adjacent the sample piece close towards the X-ray source approaches a center axis of the X-ray source.
A still further aspect of the present invention provides a fluorescent X-ray analyzer which comprises a detecting means for detecting flourescent X-rays emitted from a sample piece to be analyzed, and a first collimator comprises a plate member having a plurality of apertures defined therein and supported for movement between inserted and retracted position with respect to a path of travel of fluorescent X-rays. A second collimator is disposed between the first collimator and the detecting means and comprises a plate member having an aperture defined therein of a diameter larger than that of the apertures in the first collimator. The second collimator is supported for movement between inserted and retracted position with respect to the path of travel of the fluorescent X-rays.
According to this construction, as compared with the case in which all of the apertures of all diameters are arranged in the first collimator, the length of the collimator in a lateral direction can be shortened and, consequently, the collimator can be compactized. Also, since the first and second collimators are employed in the form of a flat plate member, the processing is easy to achieve. In addition, the smaller the target area to be measured, the more does the S/N ratio require to be improved, but this can be implemented by positioning the first collimator having the aperture of a relatively small diameter closer to the sample piece than the second collimator.